sql server select first row from a group

I have table like this:

a          b
1          23
1          2
1          7
2          9
2          11

I want to select the first row(order does not matter) from a "GROUP BY a" query , the result should be

a          b
1          23
2          9

I am using SQL SERVER 2008 how to write the query for this?

select a,b
from (
select a,b,row_number() over(partition by a order by b desc) as roworder
from myTable
) temp
where roworder = 1

see http://msdn.microsoft.com/en-us/library/ms186734.aspx

If as you indicated, order doesn't matter, any aggregate function on b would be sufficient.

Example Using MIN

SELECT a, b = MIN(b)
FROM   YourTable
GROUP BY
       a